1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a storage medium, and particularly to a method of creating a color decomposition table (to be referred to as an LUT hereinafter) to be used to convert an image signal into an ink signal for ink, toner, or the like.
2. Description of the Related Art
A conventional inkjet printing apparatus executes printing by inputting an image signal expressed by a given color space, converting the image signal into an image signal expressed by ink colors, and discharging inks based on the image signal from a printhead to a print medium. A desired color is expressed by a combination of a plurality of inks and its ink amount on the print medium, thereby forming an image. In the image signal conversion processing, a lookup table (LUT) for conversion is held in advance in accordance with a printing method and print medium, and interpolation processing is performed with reference to the grid points of the LUT.
FIG. 13 is a view showing an example of an LUT. An LUT 101 generally has a structure of a three-dimensional lookup table (3D LUT), and stores a combination of ink types and their ink signal values in each grid point 102 of the LUT 101. The grid points 102 indicate all points at which meshes intersect each other in FIG. 13, and are representative points of the LUT 101.
In general, it is possible to improve the color development by increasing the amount of information indicating the ink amount set in the LUT. However, if the ink amount is excessively increased, bleeding or overflow of inks may occur in actual printing. To avoid the occurrence of such bleeding or overflow, the limit value (maximum ink value) of the ink amount is determined in accordance with the characteristic of a print medium, an ink characteristic, a printing quality, and the like. Therefore, upon creating an LUT, it is necessary to determine an information amount indicating a combination pattern of ink types and an ink amount so as to be equal to or smaller than the set maximum ink amount limit value.
A recent user may print an image using print paper (user paper) of user's taste other than authorized paper provided by a printer maker. To meet such needs, the printer maker prepares a plurality of LUTs with different maximum ink amounts. The user can select an LUT optimum for the user paper by printing a designated chart using these LUTs, and checking bleeding/overflow of ink. Thus, as the number of LUTs provided by the printer maker is larger, a choice of LUTs is larger, thereby allowing an LUT more optimum for the user paper to be applied. However, in order for the printer maker to prepare a larger number of LUTs, an enormous time and a mass memory for storing the LUTs are required. To solve this problem, if it is possible to automatically create LUTs with different maximum ink amounts based on an LUT created in advance in which any desired maximum ink amount is set, it is possible to provide an LUT more optimum for the user paper.
There has been conventionally proposed a method of automatically generating an LUT. For example, according to Japanese Patent Laid-Open No. 2007-088636, the ink amount saturation degrees of the primary, secondary, third, and higher-order colors are calculated based on an LUT created in advance (to be referred to as a reference LUT hereinafter), its maximum ink value (to be referred to as a reference maximum value hereinafter), and the maximum value (to be referred to as the generation maximum value hereinafter) of an LUT to be automatically generated. Then, the ink amount of each grid point of the LUT is automatically generated using the saturation degrees. In addition, Japanese Patent Laid-Open No. 2010-232975 proposes a technique of automatically generating an LUT by calculating an adjustment amount based on a reference maximum value and a generation maximum value, and adjusting a reference table based on the adjustment amount.
In any of the above-described patent publications, however, the total ink amount is limited by only the maximum ink amount, and if the LUT is used to execute printing, unevenness in glossiness may occur in a printed image depending on a grid point. The cause of unevenness in glossiness will be described in detail later.